1. Field of the Invention
The present invention relates to a method of estimating an eccentric position and an acceleration generating apparatus having a function of adjusting eccentricity. More specifically, the method according to the present invention is for an acceleration sensor mounted as an object to be inspected on an auxiliary turntable (auxiliary rotating body) of a double turntable-type acceleration generating apparatus, which is suitable for investigating the characteristics of the acceleration sensor for detecting acceleration. The method is suitable for estimating an amount of eccentricity, by which the acceleration sensor is off-centered with respect to the rotational center of the auxiliary turntable. Further, the apparatus according to the present invention relates to a double turntable-type acceleration generating apparatus and a single turntable-type acceleration generating apparatus, both having an eccentricity adjusting function
2. Description of the Related Art
Conventionally, it has been the practice to detect by an acceleration sensor the acceleration of an object which moves three-dimensionally such as a robot, an airplane, or the like, and an object which moves in a two-dimensional plane such as an automobile. The movement of the object or various equipment mounted on the object is controlled in accordance with respective purposes on the basis of the result of detection. To realize this control, acceleration sensors of various forms are presently used, but in a case where an acceleration sensor is incorporated into a controlling device, it is necessary to ascertain in advance what characteristics the acceleration sensor which is used actually have.
To investigate the various characteristics of the acceleration sensor, the investigation is presently conducted by using a vibration tester, but the following problems are encountered with this method.
(1) The vibration tester investigates the acceleration characteristics by applying reciprocating acceleration to a specimen mounted on a testing table. However, there are cases where the testing table slightly tilts during the reciprocating motion (vertical motion) of the testing table in the light of the mechanism of the tester, and it is difficult to reciprocate the testing table accurately. Namely, components other than the component in the reciprocating direction act in the acceleration sensor during the test, and the acceleration is measured by including an interference output, so that this method is not sufficient to precisely investigate the various characteristics of the sensor.
(2) As for the aforementioned vibration tester, since the test is carried out by actually imparting acceleration to the acceleration sensor by using a vibration generating apparatus, the tester inevitably becomes large in size. Moreover, since the dynamic acceleration of the vibration must be actually imparted to the acceleration sensor, the performance test of the acceleration sensor is troublesome.
Against such a backdrop, to overcome the above-described problems, the present inventors proposed a novel centrifugal acceleration tester (double turntable-type acceleration generating apparatus) which is capable of improving the characteristic accuracy remarkably (refer to JP-A-7-110342). In this double turntable-type acceleration generating apparatus, an acceleration in a predetermined direction with a predetermined force (i.e., an acceleration of a DC component) is imparted by a centrifugal force to the acceleration sensor mounted on an auxiliary turntable of the apparatus, and the various characteristics of the acceleration sensor can be precisely tested.
However, in the case where the characteristics of the acceleration sensor are investigated by using the above-described turntable-type acceleration generating apparatus, it is, in practice, virtually impossible to align the rotational center of the auxiliary turntable and the center of gravity of the sensitivity axis of the acceleration sensor serving as an object to be inspected. For this reason, an output error occurs in the output signal due to the eccentricity of the rotational center of the auxiliary turntable with respect to the center of gravity of the sensitivity axis of the acceleration sensor, and there has been a need to correct this error by various methods.
Namely, when the characteristics of a commercially available acceleration are tested, it is extremely important to ascertain the center of gravity of the sensitivity axis of the acceleration sensor. At present, however, no method is available for accurately ascertaining the center of gravity of the sensitivity axis. For this reason, an error caused by the deviation of the center of gravity of the sensitivity axis is inevitably found in the output value of the characteristic of the acceleration sensor, so that it has been impossible to know accurate acceleration sensor characteristics.
For this reason, in the double turntable-type acceleration generating apparatus, a serious error occurs if the center of gravity of the sensitivity axis of the acceleration sensor cannot be set at the rotational center of the auxiliary turntable. Although the error due to eccentricity and a DC component due to the eccentricity error can be separated, it becomes impossible to measure the intrinsic low-frequency response of the acceleration sensor in the state in which the DC component is superposed. For this reason, it is necessary to align the center of gravity of the sensitivity axis of the acceleration sensor and the rotational center of the auxiliary turntable or make adjustment such that the error level is contained below measurement accuracy. For this purpose, it is, first of all, prerequisite to be able to estimate the amount of eccentricity.
Here, referring to the drawings, a brief description will be given of the effect of eccentricity on an acceleration sensor element. FIG. 9(a) shows the operation of a cantilever within an acceleration detecting unit in a case where there is no eccentricity, while FIG. 9(b) shows the operation of the cantilever within the acceleration detecting unit in a case where there is eccentricity.
Generally, the indeterminacy of the position of the center of gravity of the sensitivity axis of the acceleration sensor is 0.0008 [m] or, thereabouts at maximum, and constitutes an element of error even in the test of the single turntable system. Further, in the double turntable system, secondary centripetal acceleration occurring due to the centrifugal force of the rotation of the auxiliary turntable and acceleration due to Coriolis force are added. There is a possibility that in a case where there is an eccentricity of 0.0008 [m], acceleration of as much as 1 G or more at maximum occurs only by the rotation of the auxiliary turntable. In addition, the error due to the effect of this eccentricity produces a result in which a DC component is superposed on the low-frequency signal output of the acceleration sensor, and it is possible to separate the two components.
However, if the low-frequency acceleration is applied in the state in which a constant G is applied to the acceleration sensor, it is conceivable that the element vibrates on only the positive electrode side or the negative electrode side of the acceleration sensor, and it is conceivable that the measurement is possibly effected under a condition totally different from that of the intrinsic low-frequency response, as shown in FIG. 9 referred to above. In addition, depending on the acceleration sensor, it is conceivable that the sensitivity range is exceeded due to the superposition of the DC component.
As countermeasures against such eccentricity error, it is possible to cite the following:
(a) The eccentricity error output can be reduced to zero by rotating the main and auxiliary turntables by the same number of revolutions and in the opposite directions.
(b) The amount of eccentricity is reduced to zero or to a level at which no effect is exerted on the measurement accuracy level.
(c) The range of occurrence of frequency is made small. For example, to 1 [Hz] or less.
However, in the countermeasure based on (a), there is only one point at which the frequency can be generated with respect to each arbitrary acceleration, and even if the radius of rotation is made variable, the degree of freedom cannot be made very large. In addition, in the countermeasure based on (c), the value of the double turntable-type low-frequency acceleration generating apparatus is reduced.
From the above, as a countermeasure which is considered to be most effective at present, the technique of (b) in which the amount of eccentricity is made to approach zero is considered to be the best.
Accordingly, the present invention has been devised on the basis of the above-described knowledge, and its object is to provide a double turntable-type low-frequency acceleration generating apparatus which is capable of estimating the amount of deviation between the center of gravity of the sensitivity axis of the acceleration sensor and the rotational center of the auxiliary turntable and of adjusting the amount of eccentricity, thereby making it possible to align the center of gravity of the sensitivity axis of the acceleration sensor and the rotational center of the auxiliary turntable. In addition, this method of estimating the amount of eccentricity is naturally applicable to a single turntable-type acceleration generating apparatus as well.
In accordance with the present invention, even if the acceleration sensor is disposed with its center of gravity of the sensitivity axis off-centered at any position, the amount of its eccentricity can be easily estimated. In addition, by adjusting the position of the center of gravity of the sensitivity axis of the acceleration sensor on the basis of the estimated amount of eccentricity, it is possible to align the center of gravity of the sensitivity axis of the acceleration sensor and the rotational center of the auxiliary turntable, so that the acceleration sensor characteristics can be always ascertained accurately.
According to the present invention, the above object is accomplished by a method of estimating an eccentric position of an acceleration sensor in a double turntable-type acceleration generating apparatus including a main rotating body which is rotated by a first servo motor and an auxiliary rotating body which is mounted at an appropriate position on an outer periphery of the main rotating body in such a manner as to be rotatable by a second servo motor and on which an acceleration sensor can be mounted thereon, the method comprising the steps of: mounting the acceleration sensor, which is an object to be inspected, on the auxiliary rotating body; rotating the auxiliary rotating body at a constant frequency in a state that the main rotating body is stopped; measuring an output signal from the acceleration sensor in the state that the main rotating body is stopped; determining an amount of eccentricity in an X-axis direction of the acceleration sensor from the frequency of the auxiliary rotating body and the output signal at that time; and determining an amount of eccentricity in a Y-axis direction by using tangential acceleration generated by the rotation of the auxiliary rotating body in a state in which the amount of eccentricity in the X-axis direction is set to zero.
Further, according to the present invention, the above object is accomplished by a double turntable-type acceleration generating apparatus including a main rotating body which is rotated by a first servo motor and an auxiliary rotating body which is mounted at an appropriate position on an outer periphery of the main rotating body in such a manner as to be rotatable by a second servo motor and on which an acceleration sensor can be mounted thereon, wherein an eccentricity adjuster including an X-Y table movable in an X-direction and a Y-direction and mounted on the auxiliary rotating body.
It is also preferable that in the above double turntable-type acceleration generating apparatus, the X-Y table which is movable in the X-direction and the Y-direction is arranged to be adjusted on the basis of outputs from: a first calculator for calculating an amount of eccentricity in X-axis direction of the acceleration sensor by measuring an output signal from the acceleration sensor during rotation of the auxiliary rotating body at a constant frequency with the main rotating body held at a standstill, in which the amount of eccentricity in X-axis direction is determined from the frequency of the auxiliary rotating body and the output signal at that time; and a second calculator for calculating an amount of eccentricity in Y-axis direction of the acceleration sensor by using tangential acceleration generated by the rotation of the auxiliary rotating body in a state in which the amount of eccentricity in the X-direction is set to zero.
Further, according to the present invention, the above object is also accomplished by a single turntable-type acceleration generating apparatus including a rotating body which is rotated by a servo motor, an acceleration sensor being mountable on the rotating body; and an eccentricity adjuster including an X-Y table movable in an X-direction and a Y-direction and mounted on the rotating body.